This invention relates to an optical interferometric method and apparatus for the non-contacting measurement of the shape of a surface or structure.
Interferometric techniques for the measurement of surface shape are all based on an analysis of the interaction between a first or reference beam, normally contained within the housing of the measuring instrument, and a second or measuring beam which, after travelling from the measuring instrument to the test surface, is reflected back into the measuring instrument to interfere with the reference beam. In order to interfere, the first and second beams must be coherent; this can be arranged by using a beam splitter to derive both beams from a common source and ensuring that the difference in path length between the first and second beams does not exceed the coherence length of the source. The coherence length of the source is approximately equal to .lambda..sup.2 /.DELTA..lambda., where .lambda. is the average wavelength of the source and .DELTA..lambda. is the spectral width. Lasers can provide an extremely intense source of light with a long coherence length and are ideally suited to interferometric measurements on remote, optically rough surfaces. Indeed, with an efficient optical system design based on a focused measuring beam, it is possible to make interferometric measurements on natural engineering surfaces tens of meters distant from the measuring instrument with a measuring beam power of less than 1 mW. Laser systems limited to this power level enjoy the commercial advantage of being safe to use in an industrial laboratory with minimal disruption to normal working practices. The ease of manufacture and use of such systems is greatly enhanced if the laser beam is visible.
The shape of a remote surface may be measured by aiming the focused measuring beam of a laser interferometer sequentially at a matrix of locations on the surface. At each location, the coordinates of the point of measurement transverse to the measuring beam axis may be inferred from the beam aiming mechanism, while the distance along the measuring beam may be derived interferometrically. Known laser interferometric methods appropriate for measuring the distance to an optically rough surface make use of an optical frequency modulated source or a dual wavelength source; the round trip distance along the measuring beam is inferred from the measured frequency or phase of the interferometric modulation between the measuring beam and an internal reference beam derived from the same source. Whilst elegant in concept, such interferometric methods impose stringent requirements on the frequency stability of the laser source, the precision with which any necessary optical frequency modulation of the source can be implemented and the coherence length of the source, which must be at least as long as the maximum round trip path difference between the measuring beam and the internal reference beam.
It is an object of the present invention to obviate or mitigate these technical difficulties by providing an interferometric system based on the use of a short coherence length source without optical frequency modulation, the measuring accuracy of the system being independent of the frequency stability of the source.